Image processing device and printing apparatus for performing bidirectional printing

ABSTRACT

A printing apparatus performs printing on a print medium. The printing apparatus includes: a dot data generator that performs a halftone process on image data, wherein the print image is formed by mutually combining print pixels belonging to each of a plurality of pixel position groups for which a physical difference is assumed at a formation of dots by the print image generator, in a common print area, and the halftone process is configured to determine the status of dot formation on each of the print pixels on an assumption of the physical difference.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/350,374, filed on Feb. 7, 2006 now U.S. Pat. No. 7,965,419. Thedisclosure of this prior application is hereby incorporated by referencein its entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to technology for printing an image by formingdots on a print medium.

2. Description of the Related Art

As an output device for images created by a computer, images shot usinga digital camera, or the like, printing apparatus that print images byforming dots on a print medium are widely used. These printing apparatusprint images by forming dots on a print medium by driving the heads at asuitable timing while running the dot forming head back and forth overthe print medium. Also, among the printing apparatus, there are itemsthat form dots only during the forward scan of the dot forming head, butif dots are formed by driving the head during the backward scan inaddition to the forward scan, it is possible to print images rapidly. Inthis way, a method of printing by forming dots during forward scan andbackward scan is called bidirectional printing.

With a printing apparatus that performs bidirectional printing, whenforming during forward scan and when forming during backward scan, it isnecessary to make adjustments in advance of the dot forming timing so asnot to have displacement occur for the dot formation positions. This isdue to the following kind of reason. For example, if forming dots onlyduring forward scan of the head, one reference position near the endpart of the back and forth movement is set, and it is possible to havethe dot formation start when the head passes through the referenceposition (or at a specified timing after passing the referenceposition). To print one image, it is necessary to have the dot forminghead go back and forth a plurality of times, but if the dot formationstarts from the same position each time during the forward scan, thereis no displacement of the dot position even when forming with dividingof the back and forth movement into a plurality of times.

In comparison to this, when forming dots during the backward scan aswell, so that the formed dots start being formed from exactly the endposition of the dot line formed during forward scan, it is necessary tosuitably adjust the timing of starting formation of dots during thebackward scan each time. Of course, so that dots are formed from exactlythe end of the dot row formed during forward scan, even if the timing ofstarting dot formation during the backward scan is adjusted, if therewas a tiny difference in the head movement speed during the forward scanand the backward scan, there will be position misalignment between thedots formed during forward scan and the dots formed during backwardscan. Because of this, when performing bidirectional printing, thedemand for precision for the mechanism that moves the head back andforth becomes strict. Then, when sufficient precision cannot be secured,it becomes necessary to adjust the timing of starting dot formationduring backward scan so that displacements of the dot positions do notshow up easily. From this kind of reason, for the printing apparatusthat perform bidirectional printing, there have been proposed variousmethods for the adjustment method, with incorporation of an exclusiveadjustment mechanism for adjusting the relative timing of forming dotsduring forward scan and backward scan, and adjustment programs (e.g.Unexamined Patent No. 7-81190, Unexamined Patent No. 10-329381, and thelike).

However, high precision is demanded for this kind of adjustment, sothere is of course the problem that the adjustment mechanism andadjustment program become complex and large. Also, to performbidirectional printing, because there is demand for high precision forthe mechanism for moving the dot forming head, there is the problem thatthe head movement mechanism also tends to become complex and large.Because of this, even when the dot formation position is slightlydisplaced, by suppressing to a minimum the effect on image quality,there is demand to develop technology that will make it possible to tryto simplify the dot formation position adjustment mechanism and theadjustment program, as well as the head movement mechanism. Furthermore,this kind of problem is caused not just by the displacement in the mainscan direction for bidirectional printing, but also due to, for example,shifts in the dot formation position due to physical reasons includingmechanical errors such as Sub-scan direction displacement or time errorssuch as displacement of the ink spray timing. Furthermore, it occurs notjust due to dot formation position misalignment, but also to thedisplacement itself of the timing for forming the dots.

SUMMARY OF THE INVENTION

This invention was created to address the problems described above ofthe prior art, and its purpose is to provide technology making itpossible to suppress to a minimum the effect on image quality due to aphysical difference including a displacement of the dot formationposition.

In order to attain the above and the other objects of the presentinvention, there is provided a printing apparatus that performs printingon a print medium. The printing apparatus comprises: a dot datagenerator that performs a halftone process on image data representing atone value of each of the pixels constituting an original image todetermine a status of dot formation on each of the print pixels of theprint image to be formed on the print medium, for generating dot datarepresenting the determined status of dot formation, and a print imagegenerator that forms a dot on each of the print pixels for generating aprint image according to the dot data. The print image is formed bymutually combining print pixels belonging to each of a plurality ofpixel position groups for which a physical difference is assumed at aformation of dots by the print image generator, in a common print area.The halftone process is configured to determine the status of dotformation on each of the print pixels on an assumption of the physicaldifference.

According to the printing apparatus of this invention, for print pixelsbelonging to each of the plurality of pixel position groups for whichphysical differences are assumed, a halftone process is constituted suchthat the dot formation status on each of the print pixels for which thisphysical differences is assumed is decided, so degradation of imagequality due to this kind of physical difference, such as a shift in thedot formation position or the occurrence of low frequency noise due todisplacement of the dot formation timing, for example, can besuppressed.

The image quality degradation mechanism due to the organic relationshipbetween this kind of physical difference and halftone processing is aninsight first found by this inventor. Specifically, conventionalhalftone processing was constituted with a focus on the spatialfrequency distribution of the print image, so, for example, if therelative positions of a plurality of pixel position groups mutuallycombined in a shared printing area shift as a single body due to aphysical error of the printing apparatus, the relative positionalrelationship collapses, and there is excessive degradation of the imagequality, which was first made clear this time.

Furthermore, the inventors discovered the following phenomenon.Specifically, when there is a low frequency dense state for the dotsformed in a plurality of pixel position groups, when there isdisplacement of the dot formation timing, and overlapping with this theink drops are sprayed, at positions where dot density is high, statesoccur such as agglomerations of ink drops, excessive sheen, or abronzing phenomenon, and differences in the image occurs between thoseand positions at which the dot density is low. This image differencecauses the problem of being easily recognized as image unevenness by thehuman visual sense.

For the printing apparatus noted above, the physical differences caninclude displacement of the timing of dot formation for each of theplurality of pixel position groups, or, the physical differences caninclude a shift in the relative position of the dots for each of theplurality of pixel position groups.

In this way, physical differences have a broad meaning, of not onlyerrors in the mechanism of the printing apparatus of printing headposition measurement errors or Sub-scan feed volume measurement errors,but also, for example, being the cause of main scan direction errors dueto printing paper uplift and ink spray timing (time error) displacementor sequence.

Based on this kind of new finding, according to the invention of thisapplication, for example with the various constitutions like those shownbelow, it is possible to suppress the degradation of image quality dueto this kind of physical difference.

With the printing apparatus noted above, the halftone process can alsobe constituted such that any of the dot patterns formed on the printpixels belonging to each of the plurality of pixel position groups hasspecified characteristics.

In this way, if the dots formed on the print pixels belonging to each ofthe plurality of pixel position groups is made to have specifiedcharacteristics, in contrast to the conventional halftone processingthat depends on the relative positional relationship of the plurality ofpixel position groups, it is possible to constitute this as a halftoneprocess with a high robustness to physical differences.

Note that the specified characteristics can be decided based on thegranularity index (specifically, the index representing how easy it isfor the dots to stand out), or to have them decided as described laterbased on the correlation coefficient of the spatial frequencydistribution. Also, the specified characteristics do not absolutely haveto be provided across all the tones reproduced by this halftone process,but can also be provided for part of the tones. Here, “part of thetones” is preferably the tones with a relatively low dot density. Thisis because tones with a relatively low dot density make it easier forthe dots to stand out.

With the printing apparatus noted above, the halftone process can alsobe further constituted so that the print images which are assumed not toinclude a shift have the specified characteristics. By doing this, it ispossible to further increase the robustness to shifts.

With the printing apparatus noted above, the halftone process can alsobe constituted so that both the print images when it is assumed they donot include the shift and the printing images when it is assumed they doinclude the shift have the specified characteristics.

By doing this, it is possible to exhibit a marked effect when it ispossible to forecast a shift format.

With the printing apparatus noted above, the specified characteristicscan be either one of blue noise characteristics or green noisecharacteristics.

With the printing apparatus noted above, it is possible to have it sothat the print image generating unit has a printing head, and whileperforming the main scan of the printing head, generates a print imageby forming dots on each of the print pixels according to dot data bothduring forward scan and backward scan of the printing head, and theplurality of pixel position groups includes a first pixel position groupfor which dots are formed during the forward scan of the printing headand a second pixel position group for which dots are formed during thebackward scan of the printing head.

By doing this, it is possible to constitute the halftone process to havea high level of robustness to displacement in the main scan directionfor bidirectional printing.

With the printing apparatus noted above, it is also possible to have itso that the print image generating unit has a printing head, and whilerepeating a main scan cycle of the printing head N times (N is aninteger of 2 or more), generates a print image by forming dots on eachof the print pixels according to the dot data, and the plurality ofpixel position groups includes a plurality of pixel position groupsdivided according to the remainder from the numerical value representingthe order of the Sub-scan direction of the main scan line divided by theaforementioned N.

By doing this, it is possible to constitute a halftone process with ahigh level of robustness to displacement in the Sub-scan direction forinterlace printing which embeds the main scan with a plurality ofcycles.

With the printing apparatus noted above, it is also possible to have itso that the print image generating unit has a plurality of printingheads, and while performing the main scan of the plurality of printingheads, generates a print image by forming dots on each of the printpixels according to the dot data, and the plurality of pixel positiongroups includes a plurality of pixel position groups in charge of thedot formation by each of the plurality of printing heads.

By doing this, for printing using a plurality of printing heads, it ispossible to constitute a halftone process with a high level ofrobustness to displacement of the dot formation position between mutualprinting heads, for example.

With the printing apparatus noted above, it is also possible to have itso that the print image generating unit has a plurality of printingheads, and while performing the Sub-scan of the print medium, generatesa print image by forming dots on each of the print pixels according tothe dot data, and the plurality of pixel position groups includes aplurality of pixel position groups in charge of the dot formation byeach of the plurality of printing heads.

By doing this, it is possible to constitute the halftone process to havea high level of robustness suitable for a line printer that forms dotson each of the print pixels while performing the Sub-scan of the printmedium.

With the printing apparatus noted above, it is also possible to have itso that the dot data generating unit has a dither matrix for which athreshold value is set for each pixel, and the presence or absence ofdot formation for each of the print pixels is decided according to thetone value of each pixel that constitutes the original image and thethreshold value set for the corresponding pixel position of the dithermatrix, and the dither matrix is constituted so that each of the spatialfrequency distributions of the threshold value set for the pixelsbelonging to each of the plurality of pixel position groups and thespatial frequency distributions of the print image have a mutuallypositive correlation coefficient, or the dot data generating unit has adither matrix for which a threshold value is set for each pixel, and thepresence or absence iof dot formation for each of the print pixels isdecided according to the tone value of each pixel that constitutes theoriginal image and the threshold value set for the corresponding pixelposition of the dither matrix, and the dither matrix is constituted sothat each of the spatial frequency distributions of the threshold valueset for the pixels belonging to each of the plurality of pixel positiongroups and the spatial frequency distributions of the print image have amutual correlation coefficient of 0.7 or greater.

If this kind of dither matrix is used, a significant effect is not givento the spatial frequency distribution of the dots formed even when thephysical differences described above occur, so it is possible toconstitute a halftone process with a high level of robustness to thephysical differences described above.

With the printing apparatus noted above, it is also possible to have itso that the halftone process is constituted so that each of the spatialfrequency distributions of the dot pattern formed on the print pixelsbelonging to each of the plurality of pixel position groups and thespatial frequency distributions of the print image have a mutuallypositive correlation coefficient or the halftone process is constitutedso that each of the spatial frequency distributions of the dot patternformed on the print pixels belonging to each of the plurality of pixelposition groups and the spatial frequency distributions of the printimage have a mutual correlation coefficient of 0.7 or greater.

With this specification, “correlation coefficient” means the Pearson'sproduct moment correlation coefficient generally used as a correlationcoefficient. The Pearson's product moment correlation coefficient is onestatistical index indicating the correlation (degree of similarity)between two data strings, and taking a real number value from −1 to 1,when close to 1, this means that there is a correlation between the twodata strings, and when close to −1, it means there is a negativecorrelation. When close to 0, it means the correlation of the two datastrings is weak. A correlation coefficient of 0.7 or greater generallymeans that the correlation is strong to a degree that cannot occur as acoincidental match.

The Pearson's product moment correlation coefficient is found bydividing the covariance of the two data strings by the product of thetwo data string standard deviation. That is to say, this can also becalled the value normalized to from −1 to 1 by dividing the covarianceof the two data strings by the product of the two data string standarddeviation. With the invention of this application, the two data stringscorrelate to any two items selected from among a plurality of datastrings for which the spatial frequency distribution of each dot patternis digitized.

Note that the fact that when this invention is reliably mounted to aprinting apparatus an effect is exhibited can be confirmed using averification method for statistical engineering, for example. Thisverification method calculates the probability of a null hypothesis(this invention not mounted) occurring, and when that probability is lowto a certain degree, it is judged that the null hypothesis is mistaken,and instead, a means of supporting an alternative hypothesis (thisinvention is mounted) is used to proceed. Here, the probability that isthe criterion for judging that a null hypothesis is mistaken(significant level) is decided as a design quality assurance demanditem. By doing this, it is possible to realize design quality assurancewithout performing confirming for all the tones or colors.

In specific terms, it is possible to perform design quality assurance ofreliably mounting this invention on a printing apparatus using the kindof method described below and exhibiting an effect, for example.

(1) A sample of a specified number of gray tones for each pixel positiongroup is printed using a printing apparatus that is subject toevaluation.

(2) The spatial frequency distribution is measured for each of theprinted patterns.

(3) The mutual correlation coefficient between the measured plurality ofspatial frequency distributions is found.

(4) It is confirmed that the correlation coefficient is positive or 0.7or greater.

Here, the probability of a null hypothesis (this invention not mounted)occurring decreases as the number of gray tone samples increases.

Note that the correlation coefficient as a specified characteristic asdescribed previously does not absolutely have to be provided across allthe gradations reproduced by this halftone process, but can be providedfor part of the gradations. Here, “part of the gradations” is preferablygradations for which the dot density is relatively low. This is becausewith gradations for which the dot density is relatively low, the dotsstand out easily. In this kind of case, it is acceptable to confirm thecorrelation coefficient for consecutive gradations for which the dotdensity is relatively low, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing showing the summary of a printingsystem as the printing apparatus of this embodiment;

FIG. 2 is an explanatory drawing showing the constitution of a computeras the image processing device of this embodiment;

FIG. 3 is an explanatory drawing showing the schematic structure of thecolor printer of this embodiment;

FIG. 4 is an explanatory drawing showing an array of inkjet nozzles foran ink spray head;

FIG. 5 is a flow chart showing the flow of the image printing process ofthis embodiment;

FIG. 6 is an explanatory drawing conceptually showing an LUT referencedfor color conversion processing;

FIG. 7 is an explanatory drawing conceptually showing an example of partof a dither matrix;

FIG. 8 is an explanatory drawing conceptually showing the state ofdeciding the presence or absence of dot formation for each pixel whilereferencing the dither matrix;

FIG. 9 is an explanatory drawing showing the findings that became thebeginning of the invention of this application;

FIG. 10 is an explanatory drawing conceptually showing an example thespatial frequency characteristics of threshold values set for each pixelof the dither matrix having blue noise characteristics;

FIGS. 11(A), 11(B), and 11(C) are explanatory drawings conceptuallyshowing the sensitivity characteristics VTF for the spatial frequency ofthe visual sense that humans have;

FIGS. 12(A), 12(B), and 12(C) are explanatory drawings showing theresults of studying the granularity index of forward scan images forvarious dither matrixes having blue noise characteristics;

FIGS. 13(A) and 13(B) are explanatory drawings showing the results ofstudying the correlation coefficient between the position misalignmentimage granularity index and the forward scan image granularity index;

FIG. 14 is an explanatory drawing showing the principle of it beingpossible to suppress the image quality degradation even when dotposition misalignment occurs during bidirectional printing;

FIG. 15 is an explanatory drawing showing the degradation of imagequality due to presence or absence of dot position misalignment withimages formed using a general dither matrix;

FIG. 16 is a flow chart showing the flow of the process of generating adither matrix referenced with the tone number conversion process of thisembodiment;

FIGS. 17(A) and 17(B) are explanatory drawings showing the reason thatit is possible to ensure image quality during the occurrence of dotposition misalignment by not allowing mixing of first pixel positionsand second pixel positions within the same raster;

FIG. 18 is an explanatory drawing showing the printing status by lineprinter 200L having printing heads 251 and 252 for the first variationexample of the invention;

FIGS. 19(A) and 19(B) are explanatory drawings showing the printingstatus using the interlace recording method for the second variationexample of the invention;

FIG. 20 is an explanatory drawing showing the printing status using theoverlap recording method for the third variation example of theinvention;

FIG. 21 is an explanatory drawing showing a group of eight pixelpositions classified according to the number of remainders when the pathnumber is divided by 8;

FIGS. 22(A), 22(B), and 22(C) are is an explanatory drawing showing anexample of the actual printing status for the bidirectional printingmethod of the fourth variation example of the invention; and

FIG. 23 is an explanatory drawing showing the state of the printingimage being formed with mutually combining four pixel position groups ina common printing area in a case when conventional halftone processingwas performed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is explained in the following sequence based onembodiments.

A. Summary of the Embodiment:

B. Device Constitution:

C. Summary of the Image Printing Process:

D. Principle of Suppressing Degradation of Image Quality Due to DotPosition misalignment:

E. Dither Matrix Generating Method:

F. Variation Examples:

A. Summary of the Embodiments

Before starting the detailed description of the embodiment, a summary ofthe embodiment is described while referring to FIG. 1. FIG. 1 is anexplanatory drawing showing a summary of a printing system as theprinting apparatus of this embodiment. As shown in the drawing, theprinting system consists of a computer 10 as the image processingdevice, a printer 20 that prints the actual images under the control ofthe computer 10 and the like, and entire system is unified as one andfunctions as a printing apparatus.

A dot formation presence or absence decision module and a dither matrixare provided in the computer 10, and when the dot formation presence orabsence decision module receives image data of the image to be printed,while referencing the dither matrix, data (dot data) is generated thatrepresents the presence or absence of dot formation for each pixel, andthe obtained dot data is output toward the printer 20.

A dot formation head 21 that forms dots while moving back and forth overthe print medium and a dot formation module that controls the dotformation at the dot formation head 21 are provided in the printer 20.When the dot formation module receives dot data output from the computer10, dot data is supplied to the head to match the movement of the dotformation head 21 moving back and forth. As a result, the dot formationhead 21 that moves back and forth over the print medium is driven at asuitable timing, forms dots at suitable positions on the print medium,and an image is printed.

Also, with the printing apparatus of this embodiment, by performing socalled bidirectional printing for which dots are formed not only duringforward scan of the dot formation head 21 but also during backward scan,it is possible to rapidly print images. It makes sense that whenperforming bidirectional printing, when dot formation positionmisalignment occurs between dots formed during forward scan and dotsformed during backward scan, the image quality is degraded. In light ofthis, it is normal to have built into this kind of printer a specialmechanism or control for adjusting at a high precision the timing of dotformation of one of the back and forth movements to the other timing,and this is one factor in causing printers to be larger or more complex.

Considering this kind of point, with the printing apparatus of thisembodiment shown in FIG. 1, as the dither matrix referenced whengenerating dot data from the image data, a matrix having at least thefollowing two characteristics is used. Specifically, as the firstcharacteristic, this is a matrix for which it is possible to classifythe dither matrix pixel positions into a first pixel position group anda second pixel position group. Here, the first pixel position and thesecond pixel position are pixel positions having a relationship wherebywhen one has dots formed at either the forward scan or the backwardscan, the other has dots formed at the opposite. Then as the secondcharacteristic, this is a matrix for which the dither matrix, a matrixfor which the threshold values set for the first pixel positions areremoved from the dither matrix (first pixel position matrix), and amatrix for which the threshold values set for the second pixel positionsare removed (second pixel position matrix) all have blue noisecharacteristics.

Here, though the details are described later, the inventors of thisapplication discovered the following kind of new findings. Specifically,there is a very strong correlation between the image quality of imagesfor which the dot formation position was displaced between the forwardscan and the backward scan and the image quality of images made only bydots formed during forward scan (images obtained with only the dotsformed during the backward scan removed from the original image;hereafter called “forward scan images”), or the image quality of imagesmade only by dots formed during backward scan (images obtained with onlythe dots formed during the forward scan removed from the original image;hereafter called “backward scan images”). Then, if the image quality ofthe forward scan images or the image quality of the backward scan imagesis improved, even when dot formation position misalignment occursbetween the forward scan and the backward scan of bidirectionalprinting, it is possible to suppress degradation of image quality.Therefore, the dither matrix can be classified by the characteristicsnoted above, specifically, it is possible to classify as a first pixelposition matrix and a second pixel position matrix, and if dot data isgenerated using a dither matrix such as one for which these threematrixes have blue noise characteristics, it is possible to have boththe forward scan images and the backward images be good image qualityimages, so it is possible to suppress to a minimum the degradation ofimage quality even when there is dot formation position misalignmentduring bidirectional printing. As a result, when adjusting the dotformation timing of one of the back and forth movements to the othertiming, there is no demand for high precision, so it is possible to havea simple mechanism and control for adjustment, and thus, it is possibleto avoid the printer becoming large and complex. Following, this kind ofembodiment is described in detail.

B. Device Constitution

FIG. 2 is an explanatory drawing showing the constitution of thecomputer 100 as the image processing device of this embodiment. Thecomputer 100 is a known computer constituted by a CPU 102 as the core, aROM 104, a RAM 106 and the like being mutually connected by a bus 116.

Connected to the computer 100 are a disk controller DDC 109 for readingdata of a flexible disk 124, a compact disk 126 or the like, aperipheral device interface PIF 108 for performing transmission of datawith peripheral devices, a video interface VIF 112 for driving a CRT113, and the like. Connected to the PIF 108 are a color printer 200described later, a hard disk 118, or the like. Also, if a digital camera120 or color scanner 122 or the like is connected to the PIF 108, it ispossible to perform image processing on images taken by the digitalcamera 120 or the color scanner 122. Also, if a network interface cardNIC 110 is mounted, the computer 100 is connected to the communicationline 300, and it is possible to fetch data stored in the storage device310 connected to the communication line. When the computer 100 fetchesimage data of the image to be printed, by performing the specified imageprocessing described later, the image data is converted to datarepresenting the presence or absence of dot formation for each pixel(dot data), and output to the color printer 200.

FIG. 3 is an explanatory drawing showing the schematic structure of thecolor printer 200 of this embodiment. The color printer 200 is an inkjet printer capable of forming dots of four colors of ink includingcyan, magenta, yellow, and black. Of course, in addition to these fourcolors of ink, it is also possible to use an inkjet printer capable offorming ink dots of a total of six colors including an ink with a lowdye or pigment concentration of cyan (light cyan) and an ink with a lowdye or pigment concentration of magenta (light magenta). Note thatfollowing, in some cases, cyan ink, magenta ink, yellow ink, black ink,light cyan ink, and light magenta ink are respectively called C ink, Mink, Y ink, K ink, LC ink, and LM ink.

As shown in the drawing, the color printer 200 consists of a mechanismthat drives a printing head 241 built into a carriage 240 and performsblowing of ink and dot formation, a mechanism that moves this carriage240 back and forth in the axial direction of a platen 236 by a carriagemotor 230, a mechanism that transports printing paper P by a paper feedmotor 235, a control circuit 260 that controls the dot formation, themovement of the carriage 240 and the transport of the printing paper,and the like.

Mounted on the carriage 240 are an ink cartridge 242 that holds K ink,and an ink cartridge 243 that holds each type of ink C ink, M ink, and Yink. When the ink cartridges 242 and 243 are mounted on the carriage240, each ink within the cartridge passes through an introduction tubethat is not illustrated and is supplied to each color ink spray heads244 to 247 provided on the bottom surface of the printing head 241.

FIG. 4 is an explanatory drawing showing an array of inkjet nozzle Nzfor the ink spray heads 244 to 247. As shown in the drawing, on thebottom surface of the ink spray heads are formed four sets of nozzlearrays that spray each color of ink C, M, Y, and K, and 48 nozzles Nzper one set of nozzle arrays are arranged at a fixed nozzle pitch k.

The control circuit 260 of the color printer 200 is constituted by aCPU, ROM, RAM, PIF (peripheral device interface), and the like mutuallyconnected by a bus, and by controlling the operation of the carriagemotor 230 and the paper feed motor 235, it controls the main scanmovement and Sub-scan movement of the carriage 240. Also, when the dotdata output from the computer 100 is received, by supplying dot data tothe ink spray heads 244 to 247 to match the main scan or Sub-scanmovement of the carriage 240, it is possible to drive these heads.

The color printer 200 having the kind of hardware constitution notedabove, by driving the carriage motor 230, moves each color ink sprayhead 244 to 247 back and forth in the main scan direction, and bydriving the paper feed motor 235, moves the printing paper P in theSub-scan direction. The control circuit 260, by driving the nozzles at asuitable timing based on dot data to match the back and forth movementof the carriage 240 (main scan) and the paper feed movement of the printmedium (Sub-scan), forms suitable colored ink dots at suitable positionson the print medium. By working in this way, the color printer 200 isable to print color images on the printing paper.

Note that though the printer of this embodiment was described as a socalled inkjet printer that forms ink dots by spraying ink drops toward aprint medium, it can also be a printer that forms dots using any method.For example, the invention of this application, instead of spraying inkdrops, can also be suitably applied to a printer that forms dots byadhering each color of toner powder onto the print medium using staticelectricity, or a so called dot impact method printer.

C. Summary of the Image Printing Process

FIG. 5 is a flow chart showing the process flow of adding a specifiedimage process by the computer 100 to an image to be printed, convertingimage data to dot data expressed by the presence or absence of dotformation, supplying to the color printer 200 as control data theobtained dot data, and printing the image.

When the computer 100 starts image processing, first, it starts readingthe image data to be converted (step S100). Here, the image data isdescribed as RGB color image data, but it is not limited to color imagedata, and it is also possible to apply this in the same way for blackand white image data as well.

After reading of the image data, the resolution conversion process isstarted (step S102). The resolution conversion process is a process thatconverts the resolution of the read image data to resolution (printingresolution) at which the color printer 200 is to print the image. Whenthe print resolution is higher than the image data resolution, aninterpolation operation is performed and new image data is generated toincrease the resolution. Conversely, when the image data resolution ishigher than the printing resolution, the resolution is decreased byculling the read image data at a fixed rate. With the resolutionconversion process, by performing this kind of operation on the readimage data, the image data resolution is converted to the printingresolution.

Once the image data resolution is converted to the printing resolutionin this way, next, color conversion processing is performed (step S104).Color conversion processing is a process of converting RGB color imagedata expressed by a combination of R, G, and B tone values to image dataexpressed by combinations of tone values of each color used forprinting. As described previously, the color printer 200 prints imagesusing four colors of ink C, M, Y, and K. In light of this, with thecolor conversion process of this embodiment, the image data expressed byeach color RGB undergoes the process of conversion to data expressed bythe tone values of each color C, M, Y, and K.

The color conversion process is able to be performed rapidly byreferencing a color conversion table (LUT). FIG. 6 is an explanatorydrawing that conceptually shows the LUT referenced for color conversionprocessing. The LUT can be thought of as a three dimensional numberchart if thought of in the following way. First, as shown in FIG. 6, wethink of a color space using three orthogonal axes of the R axis, the Gaxis, and the B axis. When this is done, all the RGB image data candefinitely be displayed correlated to coordinate points within the colorspace. From this, if the R axis, the G axis, and the B axis arerespectively subdivided and a large number of grid points are set withinthe color space, each of the grid points can be thought of asrepresenting the RGB image data, and it is possible to correlate thetone values of each color C, M, Y, and K corresponding to each RGB imagedata to each grid point. The LUT can be thought of as a threedimensional number chart in which is correlated and stored the tonevalues of each color C, M, Y, and K to the grid points provided withinthe color pace in this way. If color conversion processing is performedbased on the correlation of RGB color image data and tone data of eachcolor C, M, YU, and K stored in this kind of LUT, it is possible torapidly convert RGB color image data to tone data of each color C, M, Y,and K.

When tone data of each color C, M, Y, and K is obtained in this way, thecomputer 100 starts the tone number conversion process (step S106). Thetone number conversion process is the following kind of process. Theimage data obtained by the color conversion process, if the data lengthis 1 byte, is tone data for which values can be taken from tone value 0to tone value 255 for each pixel. In comparison to this, the printerdisplays images by forming dots, so for each pixel, it is only possibleto have either state of “dots are formed” or “dots are not formed.” Inlight of this, instead of changing the tone value for each pixel, withthis kind of printer, images are expressed by changing the density ofdots formed within a specified area. The tone number conversion processis a process that, to generate dots at a suitable density according tothe tone value of the tone data, decides the presence or absence of dotformation for each pixel.

As a method of generating dots at a suitable density according to thetone values, various methods are known such as the error diffusionmethod and the dither method, but with the Tone number conversionprocess of this embodiment, the method called the dither method is used.The dither method of this embodiment is a method that decides thepresence or absence of dot formation for each pixel by comparing thethreshold value set in the dither matrix and the tone value of the imagedata for each pixel. Following is a simple description of the principleof deciding on the presence or absence of dot formation using the dithermethod.

FIG. 7 is an explanatory drawing that conceptually shows an example ofpart of a dither matrix. The matrix shown in the drawing randomly storesthreshold values selected thoroughly from a tone value range of 1 to 255for a total of 8192 pixels, with 128 pixels in the horizontal direction(main scan direction) and 64 pixels in the vertical direction (Sub-scandirection). Here, selecting from a range of 1 to 255 for the tone valueof the threshold value with this embodiment is because in addition tohaving the image data as 1 byte data that can take tone values fromvalues 0 to 255, when the image data tone value and the threshold valueare equal, it is decided that a dot is formed at that pixel.

Specifically, when dot formation is limited to pixels for which theimage data tone value is greater than the threshold value (specifically,dots are not formed on pixels for which the tone value and thresholdvalue are equal), dots are definitely not formed at pixels havingthreshold values of the same value as the largest tone value that theimage data can have. To avoid this situation, the range that thethreshold values can have is made to be a range that excludes themaximum tone value from the range that the image data can have.Conversely, when dots are also formed on pixels for which the image datatone value and the threshold value are equal, dots are always formed atpixels having a threshold value of the same value as the minimum tonevalue that the image data has. To avoid this situation, the range thatthe threshold values can have is made to be a range excluding theminimum tone value from the range that the image data can have. Withthis embodiment, the tone values that the image data can have is from 0to 255, and since dots are formed at pixels for which the image data andthe threshold value are equal, the range that the threshold values canhave is set to 1 to 255. Note that the size of the dither matrix is notlimited to the kind of size shown by example in FIG. 7, but can also bevarious sizes including a matrix for which the vertical and horizontalpixel count is the same.

FIG. 8 is an explanatory drawing that conceptually shows the state ofdeciding the presence or absence of dot formation for each pixel whilereferring to the dither matrix. When deciding on the presence or absenceof dot formation, first, a pixel for deciding about is selected, and thetone value of the image data for that pixel and the threshold valuestored at the position corresponding in the dither matrix are compared.The fine dotted line arrow shown in FIG. 8 typically represents thecomparison for each pixel of the tone value of the image data and thethreshold value stored in the dither matrix. For example, for the pixelin the upper left corner of the image data, the threshold value of theimage data is 97, and the threshold value of the dither matrix is 1, soit is decided that dots are formed at this pixel. The arrow shown by thesolid line in FIG. 8 typically represents the state of it being decidedthat dots are formed in this pixel, and of the decision results beingwritten to memory. Meanwhile, for the pixel that is adjacent at theright of this pixel, the tone value of the image data is 97, and thethreshold value of the dither matrix is 177, and since the thresholdvalue is larger, it is decided that dots are not formed at this pixel,With the dither method, by deciding whether or not to form dots for eachpixel while referencing the dither matrix in this way, image data isconverted to data representing the presence or absence of dot formationfor each pixel. In this way, if using the dither method, it is possibleto decide the presence or absence of dot formation for each pixel with asimple process of comparing the tone value of the image data and thethreshold value set in the dither matrix, so it is possible to rapidlyimplement the tone number conversion process.

Also, when the image data tone value is determined, as is clear from thefact that whether or not dots are formed on each pixel is determined bythe threshold value set in the dither matrix, with the dither method, itis possible to actively control the dot generating status by thethreshold value set in the dither matrix. With the tone numberconversion process of this embodiment, using this kind of feature of thedither method, by deciding on the presence or absence of dot formationfor each pixel using the dither matrix having the specialcharacteristics described later, even in cases when there is dotformation position misalignment between dots formed during forward scanand dots formed during backward scan when doing bidirectional printing,it is possible to suppress to a minimum the degradation of image qualitydue to this. The principle of being able to suppress to a minimum theimage quality degradation and the characteristics provided with a dithermatrix capable of this are described in detail later.

When the tone number conversion process ends and data representing thepresence or absence of dot formation for each pixel is obtained from thetone data of each color C, M, Y, and K, this time, the interlace processstarts (step S108). The interlace process is a process that realigns thesequence of transfer of image data converted to the expression formataccording to the presence or absence of dot formation to the colorprinter 200 while considering the sequence by which dots are actuallyformed on the printing paper. The computer 100, after realigning theimage data by performing the interlace process, outputs the finallyobtained data as control data to the color printer 200 (step S110).

The color printer 200 prints images by forming dots on the printingpaper according to the control data supplied from the computer 100 inthis way. Specifically, as described previously using FIG. 3, the mainscan and the Sub-scan of the carriage 240 are performed by driving thecarriage motor 230 and the paper feed motor 235, and the head 241 isdriven based on the dot data to match these movements, and ink drops aresprayed. As a result, suitable color ink dots are formed at suitablepositions and an image is printed.

The color printer 200 described above forms dots while moving thecarriage 240 back and forth to print images, so if dots are formed notonly during the forward scan of the carriage 240 but also during thebackward scan, it is possible to rapidly print images. It makes sensethat when performing this kind of bidirectional printing, when dotformation position misalignment occurs between dots formed during theforward scan of the carriage 240 and the dots formed during the backwardscan, the image quality will be degraded. In light of this, to avoidthis kind of situation, a normal color printer is made to be able toadjust with good precision the timing of forming dots for at least oneof during forward scan or backward scan. Because of this, it is possibleto match the position at which dots are formed during the forward scanand the position at which dots are formed during the backward scan, andit is possible to rapidly print images with high image quality withoutdegradation of the image quality even when bidirectional printing isperformed. However, on the other hand, because it is possible to adjustwith good precision the timing of forming dots, a dedicated adjustmentmechanism or adjustment program is necessary, and there is a tendencyfor the color printer to become more complex and larger.

To avoid the occurrence of this kind of problem, with the computer 100of this embodiment, even when there is a slight displacement of the dotformation position during the forward scan and the backward scan, thepresence or absence of dot formation is decided using a dither matrixthat makes it possible to suppress to a minimum the effect on imagequality. If the presence or absence of dot formation for each pixel isdecided by referencing this kind of dither matrix, even if there isslight displacement of the dot formation positions between the forwardscan and the backward scan, there is no significant effect on the imagequality. Because of this, it is not necessary to adjust with highprecision the dot formation position, and it is possible to use simpleitems for the mechanism and control contents for adjustment, so it ispossible to avoid the color printer from becoming needlessly large andcomplex. Following, the principle that makes this possible is described,and after that, a simple description is given of one method forgenerating this kind of dither matrix.

D. Principle of Suppressing Degradation of Image Quality Due to DotPosition Misalignment

The invention of this application was completed with the discovery ofnew findings regarding images formed using the dither matrix as thebeginning. In light of this, first, the findings we newly discovered asthe beginning of the invention of this application are explained.

FIG. 9 is an explanatory drawing showing the findings that became thebeginning of the invention of this application. Overall dot distributionDpall shows an expanded view of the state of dots being formed at aspecified density for forming images of certain tone values. As shown inOverall dot distribution Dpall, to obtain the optimal image qualityimage, it is necessary to form dots in a state dispersed as thoroughlyas possible.

To form dots in a thoroughly dispersed state in this way, it is knownthat it is possible to reference a dither matrix having so-called bluenoise characteristics to decide the presence or absence of dotformation. Here, a dither matrix having blue noise characteristics meansa matrix like the following. Specifically, it means a dither matrix forwhich while dots are formed irregularly, the spatial frequency componentof the set threshold value has the largest component in a high frequencyrange for which one cycle is two pixels or less. Note that bright (highbrightness level) images and the like can also be cases when dots areformed in regular patterns near a specific brightness level.

FIG. 10 is an explanatory drawing that conceptually shows an example ofthe spatial frequency characteristics of the threshold values set foreach pixel of a dither matrix having blue noise characteristics(following, this may also be called a blue noise matrix). Note that withFIG. 10, in addition to the blue noise matrix spatial frequencycharacteristics, there is also a display regarding the spatial frequencycharacteristics of the threshold values set in a dither matrix having socalled green noise characteristics (hereafter, this is also called agreen noise matrix). The green noise matrix spatial frequencycharacteristics will be described later, but first, the blue noisematrix spatial frequency characteristics are described.

In FIG. 10, due to circumstances of display, instead of using spatialfrequency for the horizontal axis, cycles are used. It goes withoutsaying that the shorter the cycle, the higher the spatial frequency.Also, the vertical axis of FIG. 10 shows the spatial frequency componentfor each of the cycles. Note that the frequency components shown in thedrawing indicate a state of being smoothed so that the changes aresmooth to a certain degree.

The spatial frequency component of the threshold values set for the bluenoise matrix is shown by example using the solid line in the drawing. Asshown in the drawing, the blue noise matrix spatial frequencycharacteristics are characteristics having the maximum frequencycomponent in the high frequency range for which one cycle length is twopixels or less. The threshold values of the blue noise matrix are set tohave this kind of spatial frequency characteristics, so if the presenceor absence of dot formation is decided based on a matrix having thiskind of characteristics, then dots are formed in a state separated fromeach other.

From the kinds of reasons described above, if the presence or absence ofdot formation for each pixel is decided while referencing a dithermatrix having blue noise characteristics, as shown in the Overall dotdistribution Dpall, it is possible to obtain an image with thoroughlydispersed dots. Conversely, because dots are generated dispersedthoroughly as shown in the Overall dot distribution Dpall, thresholdvalues adjusted so as to have blue noise characteristics are set in thedither matrix.

Note that here, the spatial frequency characteristics of the thresholdvalues set in the green noise matrix shown in FIG. 10 are described. Thedotted line curve shown in FIG. 10 shows an example of green noisematrix spatial frequency characteristics. As shown in the drawing, greennoise matrix spatial frequency characteristics are characteristicshaving the largest frequency component in the medium frequency range forwhich the length of one cycle is from two pixels to ten or more pixels.The green noise matrix threshold values are set so as to have this kindof spatial frequency characteristics, so when the presence or absence ofdot formation for each pixel is decided while referencing a dithermatrix having green noise characteristics, while dots are formedadjacent in several dot units, overall, the dot group is formed in adispersed state. As with a so-called laser printer or the like, with aprinter for which stable formation of fine dots of approximately onepixel is difficult, by deciding the presence or absence of dot formationwhile referencing this kind of green noise matrix, it is possible tosuppress the occurrence of isolated dots. As a result, it becomespossible to rapidly output images with stable image quality. Conversely,threshold values adjusted to have green noise characteristics are set inthe dither matrix referenced when deciding the presence or absence ofdot formation with a laser printer or the like.

As described above, with an inkjet printer like the color printer 200, adither matrix having blue noise characteristics is used, and therefore,as shown in the Overall dot distribution Dpall, the obtained image is animage with thoroughly dispersed dots. However, when this image is viewedwith the dots formed during forward scan of the head separated from thedots formed during the backward scan, we found that the images made onlyby dots formed during the forward scan (forward scan images) and theimages made only by dots formed during the backward scan (backward scanimages) do not necessarily have the dots thoroughly dispersed. Dotsformed during forward scan Dpf is an image obtained by extracting onlythe dots formed during the forward scan from the image shown in theOverall dot distribution Dpall. Also, Dots formed during backward scanDpb is an image obtained by extracting only the dots formed during thebackward scan from the image shown in the Overall dot distributionDpall.

As shown in the drawing, if the dots formed by both the back and forthmovements are matched, as shown in the Overall dot distribution Dpall,regardless of the fact that the dots are formed thoroughly, the image ofonly the dots formed during the forward scan shown in the dots formedduring forward scan Dpf or the image of only the dots formed during thebackward scan shown in the dots formed during backward scan Dpb are bothgenerated in a state with the dots unbalanced.

In this way, though it is unexpected that there would be a bigdifference in tendency, if we think in the following way, it seems thatthis is a phenomenon that occurs half by necessity. Specifically, asdescribed previously, the dot distribution status depends on the settingof the threshold values of the dither matrix, and the dither matrixthreshold values are set with special generation of the distribution ofthe threshold values to have blue noise characteristics so that the dotsare dispersed well. Here, among the dither matrix threshold values,threshold values of pixels for which dots are formed during the forwardscan or threshold values of pixels for which dots are formed during thebackward scan are taken, and with no consideration such has having thedistribution of the respective threshold values having blue noisecharacteristics, the fact that the distribution of these thresholdvalues, in contrast to the blue noise characteristics, havecharacteristics having a large frequency component in the long frequencyrange, seems half necessary (see FIG. 10). Also, for a dither matrixhaving green noise characteristics as well, when we consider that thisis a matrix specially set for the threshold value distribution to havegreen noise characteristics, the threshold values of the pixels forwhich dots are formed during the forward scan or the backward scan areconsidered to have a large frequency component on a longer cycle sidethan the cycle for which the green noise matrix has a large frequencycomponent (see FIG. 10). In the end, when the threshold values of pixelsfor which dots are formed during the forward scan or the thresholdvalues of pixels for which dots are formed during the backward scan aretaken from the dither matrix having blue noise characteristics, thedistribution of those threshold values have large frequency componentsin the Visually sensitive range. Because of this, for example, even whenimages have dots thoroughly dispersed, when only dots formed during theforward scan or only dots formed during the backward scan are removed,the obtained images respectively are considered to be images for whichthe dots have unbalance occur such as shown in the dots formed duringforward scan Dpf and the dots formed during backward scan Dpb.Specifically, the phenomenon shown in FIG. 9 is not a special phenomenonthat occurs with a specific dither matrix, but rather can be thought ofas the same phenomenon that occurs with most dither matrixes.

Considering the kind of new findings noted above and the considerationsfor these findings, studies were done for other dither matrixes as well.With the study, to quantitatively evaluate the results, an index calledthe granularity index was used. In light of this, before describing thestudy results, we will give a brief description of the granularityindex.

FIG. 11 is an explanatory drawing that conceptually shows thesensitivity characteristics VTF (Visual Transfer Function) to the visualspatial frequency that humans have. As shown in the drawing, humanvision has a spatial frequency showing a high sensitivity, and there isa characteristic of the sensitivity decreasing gradually as the spatialfrequency increases. It is also known that there is a characteristic ofthe vision sensitivity decreasing also in ranges for which the spatialfrequency is extremely low. An example of this kind of human visionsensitivity characteristic is shown in FIG. 11 (a). Various experimentalformulae have been proposed as an experimental formula for giving thiskind of sensitivity characteristic, but a representative experimentalformula is shown in FIG. 11 (b). Note that the variable L in FIG. 11 (b)represents the observation distance, and the variable u represents thespatial frequency.

Based on this kind of visual sensitivity characteristic VTF, it ispossible to think of a granularity index (specifically, an indexrepresenting how easy it is for a dot to stand out). Now, we will assumethat a certain image has been Fourier transformed to obtain a powerspectrum. If that power spectrum happens to contain a large frequencycomponent, that doesn't necessarily mean that that image willimmediately be an image for which the dots stand out. This is because asdescribed previously using FIG. 11 (a), if that frequency is in the lowrange of human visual sensitivity, for example even if it has a largefrequency component, the dots do not stand out that much. Conversely,with frequencies in the high range of human visual sensitivity, forexample even when there are only relatively low frequency components,for the entity doing the viewing, there are cases when the dots aresensed to stand out. From this fact, the image is Fourier transformed toobtain a power spectrum FS, the obtained power spectrum FS is weightedto correlate to the human visual sensitivity characteristic VTF, and ifintegration is done with each spatial frequency, then an indexindicating whether or not a human senses the dots as standing out or notis obtained. The granularity index is an index obtained in this way, andcan be calculated by the calculation formula shown in FIG. 11 (c). Notethat the coefficient K in FIG. 11 (c) is a coefficient for matching theobtained value with the human visual sense.

To confirm that the phenomenon described previously using FIG. 9 is nota special phenomenon that occurs with a specific dither matrix, butrather occurs also with most dither matrixes, the following kind ofstudy was performed on various dither matrixes having blue noisecharacteristics. First, from among the dots formed by bidirectionalprinting, images made only by dots formed during the forward scan suchas shown in the dots formed during forward scan Dpf (forward scanimages) are obtained. Next, the granularity index of the obtained imagesis calculated. This kind of operation was performed for various dithermatrixes while changing the image tone values.

FIG. 12 is an explanatory drawing showing the results of studying thegranularity index of forward scan images for various dither matrixeshaving blue noise characteristics. Shown in FIG. 12 are only the resultsobtained for three dither matrixes with different resolutions. Thedither matrix A shown in FIG. 12 (a) is a dither matrix for printing ata main scan direction resolution of 1440 dpi and Sub-scan directionresolution of 720 dpi, and the dither matrix B shown in FIG. 21 (b) is adither matrix used for printing at a resolution of 1440 dpi for both themain scan direction and the Sub-scan direction. Also, the dither matrixC shown in FIG. 12 (c) is a dither matrix for printing in the main scandirection at a resolution of 720 dpi and in the Sub-scan direction at aresolution of 1440 dpi. Note that in FIG. 12, the horizontal axis isdisplayed using the small dot formation density, and the areas for whichthe displayed small dot formation density is 40% or less correlate toareas up to before the intermediate gradation area from the highlightarea for which it is considered that the dots stand out relativelyeasily.

Regardless of the fact that the three forward scan images shown in FIG.12 are generated from individually created dither matrixes for printingrespectively at different resolutions, each has an area for which thegranularity index is degraded (specifically, an area in which the dotsstand out easily). In this kind of area, the forward scan image can bethought of as the dots generating imbalance as shown in the dots formedduring forward scan Dpf. In the end, all of the three dither matrixesshown in FIG. 12 have blue noise characteristics, and therefore,regardless of the fact that the images formed using bidirectionalprinting have dots formed without imbalance, in at least part of thegradation area, the forward scan image or the backward scan image hasdot imbalance occur. From this, the phenomenon described previouslyusing FIG. 9 can be thought of not as a special phenomenon that occurswith a specific dither matrix but rather as a general phenomenon thatoccurs with most dither matrixes. Then, when we consider the occurrenceof dot imbalance with either forward scan images or backward scan imagesin this way, this can be thought of as possibly having an effect on theimage quality degradation due to dot position misalignment duringbidirectional printing. In light of this, we tried studying to seewhether or not any kind of correlation can be seen between thegranularity index of images formed with an intentional displacement inthe dot formation position during bidirectional printing (positionmisalignment image) and the granularity index of forward scan images.

FIG. 13 is an explanatory drawing showing the results of studying thecorrelation coefficient between the position misalignment imagegranularity index and the forward scan image granularity index. FIG. 13(a) shows the results of a study on the dither matrix A shown in FIG. 12(a), and in the drawing, the black circles represent the positionmisalignment image granularity index and the white circles in thedrawing represent the granularity index for the forward scan image.Also, FIG. 13 (b) shows the results of a study on the dither matrix Bshown in FIG. 12 (b), and the black squares represent the positionmisalignment image granularity index while the white squares representthe forward image granularity index. As is clear from FIG. 13, for anyof the dither matrixes, a surprisingly strong correlation is seenbetween the position misalignment image granularity index and theforward image granularity index. From this fact, for the phenomenon ofthe image quality being degraded by the dot position misalignment duringbidirectional printing, the fact that the bidirectional image dotimbalance becomes marked due to displacement of the relative positionbetween the forward scan images and the backward scan images can beconsidered to be one significant factor. Conversely, if the dotimbalance between the forward scan images and the backward scan imagesis reduced, for example even when dot position misalignment occursduring bidirectional printing, it is thought that it is possible tosuppress image quality degradation.

FIG. 14 is an explanatory drawing showing that it is possible tosuppress the image quality degradation when dot position misalignmentoccurs during bidirectional printing if the dot imbalance is reduced forimages during forward scan and images during backward scan. Dot patternDat and dot pattern Dmat show a comparison of an image for whichbidirectional printing was performed in a state without dot positionmisalignment and an image printed in a state with intentionaldisplacement by a specified volume of the dot formation position. Also,shown respectively in FIG. 14, Forward scan image Fsit and Backward scanimage Bsit are images obtained by breaking down into an image made onlyby dots formed during the forward scan of the head (forward scan image)and an image made only by dots formed during the backward scan (backwardscan image).

As shown in the forward scan image Fsit and the backward scan imageBsit, the forward scan images and the backward scan images are bothimages for which the dots are dispersed thoroughly. Also, as shown inthe forward scan image Fsit, in the state with no dot positionmisalignment, images obtained by synthesizing the forward scan imagesand backward scan images (specifically, images obtained withbidirectional printing) are also images for which the dots are dispersedthoroughly. In this way, not just images obtained by performingbidirectional printing, but also when broken down into forward scanimages and backward images, images that have the dots dispersedthoroughly with the respective images can be obtained by deciding thepresence or absence of dot formation while referencing a dither matrixhaving the kind of characteristics described later in the tone numberconversion process of FIG. 5. Then, the backward scan image Bsitcorrelates to an image for which this kind of forward scan image andbackward scan image are overlapped in a state displaced by a specifiedamount.

If the image without position misalignment (left side image) shown inthe forward scan image Fsit and the image with position misalignment(right side image) are compared, by the dot position being displaced,the right side image has its dots stand out slightly more easily thanthe left side image with no displacement, but we can understand thatthis is not at a level that greatly degrades the image quality. This isthought to show that even when broken down into forward scan images andbackward scan images, if dots are generated so that the dots aredispersed thoroughly, for example even when dot position misalignmentoccurs during bidirectional printing, it is possible to greatly suppressdegradation of image quality due to this.

As a reference, with the image formed using a typical dither matrix, wechecked to what degree image quality degraded when dot positionmisalignment occurred by the same amount as the case shown in FIG. 14.FIG. 15 is an explanatory drawing showing degradation of the imagequality due to the presence or absence of dot position misalignment withthe image formed by a typical dither matrix. The image without positionmisalignment (left side image) shown in Dot pattern Dar is an image forwhich the forward scan image and backward scan image shown in FIG. 9 areoverlapped without any position misalignment. Also, the image withposition misalignment shown in Dot pattern Dar is an image for which theforward scan image and the backward scan image are overlapped in a statewith the position displaced by the same amount as the case shown in FIG.14. Note that in the forward scan image Fsir and the backward scan imageBsir, the respective forward scan images and backward scan images areshown.

As is clear from FIG. 15, when dots are generated with imbalance withthe forward scan image and the backward scan image, it is possible toconfirm that when the dot formation positions are displaced duringbidirectional printing, there is great degradation of the image qualitywhen the image quality is greatly degraded [sic]. Also, when FIG. 14 andFIG. 15 are compared, by thoroughly dispersing the dots with the forwardscan image and the backward scan image, it is possible to understandthat the image quality degradation due to dot position misalignment canbe dramatically improved.

With the color printer 200 of this embodiment, based on this kind ofprinciple, it is possible to suppress to a minimum the image qualitydegradation due to dot position misalignment during bidirectionalprinting. Because of this, during bidirectional printing, even when theformation positions of the dots formed during forward scan and the dotsformed during backward scan are not matched with high precision, thereis no degradation of image quality. As a result, there is no need for amechanism or control program for adjusting with good precision the dotposition misalignment, so it is possible to use a simple constitutionfor the printer. Furthermore, it is possible to reduce the precisionrequired for the mechanism for moving the head back and forth as well,and this point also makes it possible to simplify the printerconstitution.

E. Dither Matrix Generating Method

Next, a simple description is given of an example of a method ofgenerating a dither matrix to be referenced by the tone numberconversion process of this embodiment. Specifically, with the tonenumber conversion process of this embodiment, for dots formed during theforward scan, for dots formed during the backward scan, and furthermore,for combinations of these dots, dots are generated in a thoroughlydispersed state, so gradation conversion processing is performed whilereferencing a dither matrix having the following two kinds ofcharacteristics.

“First Characteristic”: The dither matrix pixel positions can beclassified into first pixel position groups and second pixel positiongroups. Here, the first pixel position and the second pixel positionmean pixel positions having a mutual relationship such that when dotsare formed by either the forward scan or the backward scan, the otherhas dots formed by the other.

“Second Characteristic”: The dither matrix and a matrix for which thethreshold values set for the first pixel position are removed from thatdither matrix (first pixel position matrix), and a matrix for which thethreshold values set for the second pixel positions are removed (secondpixel position matrix) all have either blue noise characteristics orgreen noise characteristics. Here, a “dither matrix having blue noisecharacteristics” means the following kind of matrix. Specifically, itmeans a dither matrix for which dots are generated irregularly and thespatial frequency component of the set threshold values have the largestcomponent in the medium frequency range for which one cycle is from twopixels to ten or more pixels. Also, a “dither matrix having green noisecharacteristics” means a dither matrix for which dots are formedirregularly and the spatial frequency component of the set thresholdvalues have the largest component in the medium frequency range forwhich one cycle has from two pixels to ten or more pixels. Note that ifthese dither matrixes are near a specific brightness, it is alsoacceptable if there are dots formed in a regular pattern.

As described previously, dither matrixes having these kind ofcharacteristics can definitely not be generated by coincidence, so abrief description is given of an example of a method for generating thiskind of dither matrix.

FIG. 16 is a flow chart showing the flow of the process of generatingdither matrixes referenced with the tone number conversion process ofthis embodiment. Note that here, with an existing dither matrix havingblue noise characteristics as a source, so that the “firstcharacteristics” and “second characteristics” described above can beobtained, described is a method to which correction is added. It makessense that rather than correcting the matrix that is the source, that itis also possible to generate first from a dither matrix having the“first characteristics” and “second characteristics.” Also, here,described is a case when a matrix having blue noise characteristics isthe source, but it is also possible to obtain a dither matrix having thecharacteristics noted above by working in about the same manner whenusing a dither matrix having green noise characteristics as the sourceas well.

When the dither matrix generating process starts, first, the dithermatrix that is the source is read (step S200). This matrix overall hasblue noise characteristics, but the first pixel position matrix (thematrix for which the threshold values set at the first pixel positionare removed from the dither matrix) and the second pixel position matrix(the matrix for which the threshold values set at the second pixelposition are removed from the dither matrix) are both matrixes that donot have blue noise characteristics. Note that as described previously,the first pixel position and the second pixel position mean pixelpositions in a mutual relationship for which when dots are formed eitherduring forward scan or backward scan, the other has dots formed by theother.

Next, the read matrix is set as matrix A (step S202). Then, from thedither matrix A, two pixel positions (pixel position P and pixelposition Q) are randomly selected (step S204), the threshold value setat the selected pixel position P and the threshold value set at theselected pixel position Q are transposed, and the obtained matrix isused as matrix B (step S206).

Next, the granularity evaluation value Eva for the matrix A iscalculated (step S208). Here, the granularity evaluation value means anevaluation value obtained as follows. First, using the dither method on256 images of tone values 0 to 255, 256 images are obtained expressed bythe presence or absence of dot formation. Next, each image is brokendown into forward scan images and backward scan images. As a result, foreach of the tone values from 0 to 255, obtained are the forward scanimage, the backward scan image, and an image for which these areoverlapped (total image). For the 768 (=256×3) images obtained in thisway, after calculation of the granularity index described previouslyusing FIG. 11, the value obtained by finding the average value of theseis used as the granularity evaluation value. Note that when calculatingthe granularity evaluation value, rather than simply using an arithmeticmean of the 768 granularity indices, it is also possible to take aweighted average respectively of the forward scan image, the backwardscan image, and the total image. Alternatively, for a specific tonevalue (e.g. a low tone range for which it is said that dots stand outrelatively easily), it is also possible to apply a large weightingcoefficient and take the average. At step S208 of FIG. 16, for thematrix A, this kind of granularity evaluation value is found, and theobtained value is used as the granularity evaluation value Eva.

When the granularity evaluation value Eva is obtained for the matrix A,the granularity evaluation value Evb is calculated in the same mannerfor the matrix B as well (step S210). Next, the granularity evaluationvalue Eva for the matrix A and the granularity evaluation value Evb forthe matrix B are compared (step S212). Then, when it is determined thatthe granularity evaluation value Eva is bigger (step S212: yes), thematrix B for which the threshold values set in the two pixel positionsare transposed is through to have more desirable characteristics thanthe matrix A which is the source. In light of this, in this case, thematrix B is reread as matrix A (step S214). Meanwhile, when it isdecided that the granularity evaluation value Evb of the matrix B islarger than the granularity evaluation value Eva of the matrix A (stepS212: no), then matrix is not reread.

In this way, only in the case when it is determined that the granularityevaluation value Eva of the matrix A is larger than the granularityevaluation value Evb of the matrix B, when the operation of rereadingthe matrix B as the matrix A, a determination is made of whether or notthe granularity evaluation values are converged (step S216).Specifically, the dither matrix set as the source has the dots formedduring the forward scan and the dots formed during the backward scangenerated with imbalance, so immediately after starting the kind ofoperation noted above, a large value is taken for the granularityevaluation value. However, by transposing the threshold values set inthe two pixel position locations, when a smaller granularity evaluationvalue is obtained, if the matrix for which the threshold value istransposed is used, and the operation described above is furtherrepeated for this matrix, the obtained granularity evaluation valuebecomes smaller, and it is thought that over time it becomes stable at acertain value. At step S216, a determination is made of whether or notthe granularity evaluation value has stabilized, or said another way,whether or not it can be thought of as having reached bottom. Forwhether or not the granularity evaluation values have converged, forexample, when the granularity evaluation value Evb of the matrix B issmaller than the granularity evaluation value Eva of the matrix A, thedecrease volume of the granularity evaluation value is obtained, and ifthis decrease volume is a fixed value or less that is stable across aplurality of operations, it can be determined that the granularityevaluation values have converged.

Then, when it is determined that the granularity evaluation values havenot converged (step S216: no), the process backwards to step S204, andafter selecting two new pixel positions, the subsequent series ofoperations is repeated. While repeating this kind of operation, overtime, the granularity evaluation values converge, and when it isdetermined that the granularity evaluation values have converged (stepS216: yes), the matrix A at that time becomes a dither matrix having thepreviously described “first characteristics” and “secondcharacteristics.” In light of this, this matrix A is stored (step S218),and the dither matrix generating process shown in FIG. 16 ends.

If tone number conversion processing is performed while referencing adither matrix obtained in this way, and a decision is made on thepresence or absence of dot formation for each pixel, it goes withoutsaying for the overall image, as well as for the forward scan images andthe backward scan images, that it is possible to obtain images for whichthe dots are dispersed well. Because of this, for example even whenthere is slight displacement of the dot formation positions duringbidirectional printing, it is possible to suppress to a minimum theeffect on the image quality by this.

Note that with this embodiment, the granularity evaluation value Evaused to evaluate the dither matrix is calculated based on thegranularity index that is the subjective evaluation value that uses thevisual sensitivity characteristic VTF, but it is also possible tocalculate based on the RMS granularity that is the standard deviation ofthe density distribution, for example.

The granularity index is a well known method and is an evaluation indexused widely from the past. However, calculation of the granularityindex, as described previously, means obtaining the power spectrum FS bydoing Fourier transformation of an image, and it is necessary to add aweighting to the obtained power spectrum FS that correlates to the humanvisual sensitivity characteristics VTF, so there is the problem of thecalculation volume becoming very large. Meanwhile, the RMS granularityis an objective measure representing variance of dot denseness, and thiscan be calculated simply just by the smoothing process using a smoothingfilter set according to the resolution and calculation of the standarddeviation of the dot formation density, so it is perfect foroptimization processing which has many repeated calculations. Inaddition, use of the RMS granularity has the advantage of flexibleprocessing being possible considering the human visual sensitivity andvisual environment according to the design of the smoothing filter incomparison to the fixed process that uses the human visual sensitivitycharacteristics VTF.

Also, with the embodiment described above, the first pixel position andthe second pixel position were described as pixel positions having amutual relationship whereby when dots are formed by either of theforward scan or the backward scan, with the other, dots are formed bythe other. Specifically, even within a row of pixels aligned in the mainscan direction (this kind of pixel alignment is called a “raster”),there are cases when a first pixel position and a second pixel positionare included. However, from the perspective of securing image qualityduring occurrence of dot position misalignment, it is preferable thatthe first pixel positions and the second pixel positions not be mixedwithin the same raster. Following is a description of the reason forthis.

FIG. 17 is an explanatory drawing showing the reason that it is possibleto ensure image quality when dot position misalignment occurs by notmixing the first pixel positions and the second pixel positions withinthe same raster. The black circles shown in the drawing indicate dotsformed during the forward scan, and the black squares indicate dotsformed during the backward scan. Specifically, if one of the blackcircles or black squares is set as the first pixel position, then theother is set as the second pixel position. FIG. 17 (a) represents astate in which the first pixel position and the second pixel positionare mixed in the same raster, and FIG. 17 (b) represents a state inwhich the first pixel position and the second pixel position are notmixed in the same raster. Also, in the respective drawings, the drawingshown at the left side indicates an image in a state without dotposition misalignment, and the drawing at the right side indicates animage in a state with dot position misalignment. As is clear from FIG.17 (a), when the first pixel position and the second pixel positions aremixed in the same raster, when dot position misalignment occurs, by thedistance between dots within the raster occurring at close locations andat distant locations, this degrades the image quality. In comparison tothis, as shown in FIG. 17 (b), if the first pixel position and thesecond pixel position are not mixed in the same raster, for example,even when dot position misalignment occurs, there is no occurrence ofthe dot distance in a raster being at close locations and distantlocations, and it is possible to suppress degradation of the imagequality.

In addition, as shown in FIG. 17 (b), if the first pixel positionrasters and the second pixel position rasters are arranged alternately,for example, even when dot position misalignment occurs, the dots aredisplaced in one direction across the subsequent rasters, and it ispossible to avoid having this visually recognized, degrading the imagequality.

As described above, the first pixel position dither matrix and thesecond pixel position dither matrix are dither matrixes having bluenoise characteristics (or green noise characteristics), and in addition,if the first pixel positions and the second pixel positions are made notto be mixed within the same raster, for example even if the dotformation positions are displaced during bidirectional printing, it ispossible to more effectively suppress this from causing degradation ofthe image quality.

F. Variation Examples:

Above, a number of embodiments of the invention were described, but theinvention is in no way limited to these kinds of embodiments, and it ispossible to embody various aspects in a scope that does not stray fromthe key points. For example, the following kinds of variation examplesare possible.

F-1. First Variation Example:

FIG. 18 is an explanatory drawing showing the printing state using aline printer 200L having a plurality of printing heads 251 and 252 forthe first variation example of the invention. The printing head 251 andthe printing head 252 are respectively arranged in a plurality at theupstream side and the downstream side. The line printer 200L is aprinter that outputs at high speed by performing only Sub-scan feedwithout performing the main scan.

Shown at the right side of FIG. 18 is a dot pattern 500 formed by theline printer 200L. The numbers 1 and 2 inside the circles indicate thatit is the printing head 251 or 252 that is in charge of dot formation.In specific terms, dots for which the numbers inside the circle are 1and 2 are respectively formed by the printing head 251 and the printinghead 252.

Inside the bold line of the dot pattern 500 is an overlap area at whichdots are formed by both the printing head 251 and the printing head 252.The overlap area makes the connection smooth between the printing head251 and the printing head 252, and is provided to make the difference inthe dot formation position that occurs at both ends of the printingheads 251 and 252 not stand out. This is because at both ends of theprinting heads 251 and 252, the individual manufacturing differencebetween the printing heads 251 and 252 is big, and the dot formationposition difference also becomes bigger, so there is a demand to makethis not stand out clearly.

In this kind of case as well, the same phenomenon as when the dotformation position is displaced between the forward scan and thebackward scan as described above occurs due to the error in the mutualpositional relationship of the printing heads 251 and 252, so it ispossible to try to improve image quality by performing the same processas the embodiment described previously using the pixel position groupformed by the printing head 251 and the pixel position group formed bythe printing head 252.

F-2. Second Variation Example:

FIG. 19 is an explanatory drawing showing the state of printing usingthe interlace recording method for the second variation example of theinvention. The interlace recording method means a recording method usedwhen the nozzle pitch k “dots” are 2 or greater measured along theSub-scan direction of the printing head. With the interlace recordingmethod, a raster line that cannot be recorded between adjacent nozzleswith one main scan is left, and the pixels on this raster line arerecorded during another main scan. With this variation example, the mainscan is also called a pass.

FIG. 19 (A) shows an example of the Sub-scan feed when using fournozzles, and FIG. 19 (B) shows the parameters of that dot recordingmethod. In FIG. 19 (A), the solid line circles containing numbersindicate the Sub-scan direction position of the four nozzles for eachpass. Here, “pass” means one main scan. The numbers 0 to 3 in thecircles mean the nozzle numbers. The position of the four nozzles issent in the Sub-scan direction each time one main scan ends.

As shown at the left end of FIG. 19 (A), with this example, the Sub-scanfeed volume L is a fixed value of four dots. Therefore, each time aSub-scan feed is performed, the four nozzle positions are displaced inthe Sub-scan direction four dots at a time. Each nozzle has as arecording subject all the dot positions (also called “pixel positions”)on the respective raster lines in one main scan. At the right end ofFIG. 19 (A) is shown the number of the nozzle that records the dots oneach raster line.

In FIG. 19 (B) are shown the various parameters relating to this dotrecording method. Included in the parameters of the dot recording methodare nozzle pitch k [dots], used nozzle count N [units], and Sub-scanfeed volume L [dots]. With the example in FIG. 19, the nozzle pitch k isthree dots. The used nozzle count N is four units.

Shown in the table in FIG. 19 (B) are the Sub-scan feed volume L foreach pass, the cumulative value ΣL thereof, and the nozzle offset F.Here, the offset F is a value that, when a reference position is assumedfor which the offset is 0 for a cyclical position of the nozzles for thefirst pass 1 (in FIG. 19, the position at every four dots), indicates byhow many dots the nozzle position for each pass after that is separatedin the Sub-scan direction from the reference position. For example, asshown in FIG. 19 (A), after pass 1, the nozzle position moves in theSub-scan direction by an amount Sub-scan feed volume L (four dots).Meanwhile, the nozzle pitch k is three dots. Therefore, the offset F ofthe nozzles for pass 2 is 1 (see FIG. 19 (A)). Similarly, the nozzleposition for pass 3 is ΣL=8 dots moved from the initial positions, andthe offset F is 2. The nozzle position for pass 4 is ΣL=12 dots movedfrom the initial position, and the offset F is 0. With pass 4 afterthree Sub-scan feeds, the nozzle offset F backwards to 0, so with threeSub-scans as one cycle, by repeating this cycle, it is possible torecord all the dots on the raster line in an effective recording range.

In this way, with the second variation example, in contrast to embeddingthe dots with the forward scan and backward scan as described above,dots are embedded with one cycle three passes, so it is conceivable thatthere will be displacement of mutual positions between each pass in onecycle due to Sub-scan feed error. Because of this, it is possible thatthe same phenomenon will occur as when the dot formation positions aredisplaced with the forward scan and backward scan described above, so itis possible to try to improve the image quality using the same processas the embodiments described above with a pixel position group formedwith the first pass of each cycle, a pixel position group formed withthe second pass, and a pixel position group formed with the third pass.

Note that with the interlace recording method, each cycle does notnecessarily embed dots with three passes, and it is also possible toconstitute one cycle with two times or four times or more. In this case,it is possible to do group division for each pass that constitutes eachcycle.

Also, the group division does not necessarily have to be performed onall the passes that constitute each cycle, and for example, it is alsopossible to constitute this to be divided into a pixel position groupformed with the last pass of each cycle for which Sub-scan feed erroraccumulation is anticipated and a pixel position group formed with thefirst pass of each cycle.

F-3. Third Variation Example:

FIG. 20 is an explanatory drawing showing the state of printing using anoverlap recording method for the third variation example of theinvention. In FIG. 20, the solid line circles including numbers indicatepositions in the Sub-scan direction of six nozzles for each pass. Thenumbers 1 to 8 in the solid line circles are the number of remaindersafter dividing the pass number by 8. The pixel position number indicatesthe sequence of the arrangement of pixels on each raster line.

The overlap recording method is a recording method for which each rasterline is formed by a plurality of passes. With the third variationexample, each raster line is formed with two passes. In specific terms,for example, the raster line for which the raster number is 1 is formedby pass 1 and pass 5, and the raster lines 2 and 3 are respectivelyformed by pass 8 and pass 4, and pass 3 and pass 7.

As can be seen from FIG. 20, the dot pattern constituted by the rasterlines for which the raster numbers are 1 to 4 are formed by eight passesof pass 1 to pass 8, and the dot pattern constituted by the raster linesfor which the raster numbers are 5 to 8 are formed by eight passes ofpass 3 to pass 10. Furthermore, when we focus on the number ofremainders when the pass number is divided by 8, by repeating the dotpattern constituted by the dots formed on pixels 1 to 4 by the rasternumber and pixel position numbers 1 to 4, we can see that all the dotpatterns are formed.

FIG. 21 is an explanatory drawing showing the eight pixel positiongroups divided according to the number of remainders when the passnumber is divided by 8. With FIG. 21, each square shape indicates animage area constituted by pixels for which the pixel position number is1 to 4 of the raster lines for which the raster number is 1 to 4. Thisimage area correlates to the “shared printing area” in the patentclaims, and is constituted by combining the print pixels belonging toeach of the eight pixel position groups.

In this kind of case as well, the same phenomenon occurs as when thereis mutual displacement of the dot positions formed with each pass, so itis possible to attempt to improve the image quality by performing thesame process as the embodiments described above so that the dots formedby each of the eight pixel position groups has specifiedcharacteristics.

F-4. Fourth Variation Example:

FIG. 22 is an explanatory drawing showing an example of the actualprinting state for the bidirectional printing method of the thirdvariation example of the invention. The letters in the circles indicatewhich of the forward or backward main scans the dots were formed with.FIG. 22 (a) shows the dot pattern when displacement does not occur inthe main scan direction. FIG. 22 (b) and FIG. 22 (c) show the dotpatterns when displacement does occur in the main scan direction.

With FIG. 22 (b), in relation to the position of dots formed at theprint pixels belonging to the pixel position group for which dots areformed during the forward movement of the printing head, the position ofthe dots formed at the print pixels belonging to the pixel positiongroup for which dots are formed during the backward scan of the printinghead is shifted by 1 dot pitch in the rightward direction. Meanwhile,with FIG. 22 (c), in relation to the position of the dots formed at theprint pixels belonging to the pixel position group for which dots areformed during the forward scan of the printing head, the position of thedots formed at the print pixels belonging to the pixel position groupfor which dots are formed during the backward scan of the printing headis shifted by 1 dot pitch in the leftward direction.

With the embodiments described above, by giving blue noise or greennoise spatial frequency distribution to both the dot patterns of thepixel position group for which dots are formed during the forward scanand the dot patterns of the pixel position group for which dots areformed during the backward scan, image quality degradation due to thiskind of displacement is suppressed.

In contrast to this, the third variation example is constituted so thatthe dot pattern for which the dot pattern formed on the pixel positiongroup formed during the forward scan and the dot pattern formed on thepixel position group formed during the backward scan are shifted by 1dot pitch in the main scan direction and synthesized has blue noise orgreen noise spatial frequency distribution, or has a small granularityindex.

The constitution of the dither matrix focusing on the granularity indexcan be constituted so that, for example, the average value of thegranularity index when the displacement in the main scan direction isshifted by 1 dot pitch in one direction, when it is shifted by 1 dotpitch in the other direction, and when it is not shifted, is a minimum.Alternatively, it is also possible to constitute this such that thespatial frequency distributions in these cases have a mutually highcorrelation coefficient.

Note that this variation example is able to increase the robustnesslevel of the image quality in relation to displacement of the dotformation position during forward scan and backward scan, so it ispossible to suppress the degradation of image quality not only in caseswhen the dot formation positions are shifted as a mass during theforward scan and the backward scan, but also when unspecifieddisplacement occurs with part of the pixel position group for which dotsare formed during the forward scan and the pixel position group forwhich dots are formed during the backward scan. For example, it ispossible to suppress degradation of the image quality also in cases suchas when there is partial variation in the gap of the printing head andthe printing paper between the forward scan and the backward scan due tocyclical deformation due to the main scan of the main scan mechanism ofthe printing head, for example.

F-5. This invention can also be applied to printing that performsprinting using a plurality of printing heads. In specific terms, it isalso possible to constitute this so that the spatial frequencydistributions of dots formed in a plurality of pixel position groups incharge of dot formation by each of the plurality of printing heads havea mutually high correlation coefficient.

By working in this way, for printing using the plurality of printingheads, it is possible to constitute halftone processing with a highrobustness level to displacement of dot formation positions betweenmutual printing heads, for example.

F-6. With this invention, the inventors found not only robustness inrelation to dot formation position misalignment, but also suppression ofdegradation of image quality due to the dot formation time sequence (ordot formation timing displacement).

FIG. 23 is an explanatory drawing showing the state of print imagesbeing formed by mutually combining in a shared printing area four imagegroups in a case when conventional halftone processing is performed.FIG. 23 shows the dot patterns when the four to one pixel positiongroups are respectively combined.

With conventional halftone processing, processing is performed with afocus on the print image dot dispersion properties formed by all thepixel position groups, so as can be seen from FIG. 23, there isunevenness in the dot dispersion properties of each pixel positiongroup. Specifically, a dense dot state occurs in the low frequency area.This kind of dense dot state causes a state of accumulation of inkdrops, excessive sheen, and a bronzing phenomenon at the positions wherethe dot density is high, and causes image differences with positions atwhich dot density is low. This image difference causes the problem of itbeing easy for the human visual sense to recognize this as imageunevenness.

This invention suppresses excessive high density of dots and reduces thestates of accumulation of ink drops, excessive sheen, and the bronzingphenomenon, and causes uniformity for the overall print image, so it isable to suppress image unevenness. In this way, this invention is ableto be applied broadly to printing that forms print images by mutuallycombining in a common print area print pixels belonging to each of aplurality of pixel position groups, and even if mutual displacement ofdots formed in the plurality of pixel position groups is not assumed, itcan be applied also in cases when there is a difference in timing offormation of dots formed in the plurality of pixel position groups. Thisinvention generally can be applied in cases when, for dot formation,print pixels belonging to each of the plurality of pixel position groupsfor which a physical difference is assumed such as displacement of timeor formation position are mutually combined in a common print area toform a print image.

F-7. With the embodiments described above, halftone processing wasperformed using a dither matrix, but it is also possible to use thisinvention in cases when halftone processing is performed using errordiffusion, for example. Using error diffusion can be realized by havingerror diffusion processing performed for each of a plurality of pixelposition groups, for example.

Note that with the dither method of the embodiments noted above, bycomparing for each pixel the threshold values set in the dither matrixand the tone values of the image data, the presence or absence of dotformation is decided for each pixel, but it is also possible to decidethe presence or absence of dot formation by comparing the thresholdvalues and the sum of the tone values with a fixed value, for example.Furthermore, it is also possible to decide the presence or absence ofdot formation according to the data generated in advance based onthreshold value as and on the tone values without directly using thethreshold values. The dither method of this invention generally can be amethod that decides the presence or absence of dot formation accordingto the tone value of each pixel and the threshold value set for thepixel position corresponding to the dither matrix.

This invention also includes the following configuration as examples.With the printing apparatuses described above, it is also possible tohave it so that the halftone process is constituted so that, at leastfor the tone level with relatively low dot density, the correlationcoefficient between each of the spatial frequency distributions of thedot pattern formed on the print pixels belonging to each of theplurality of pixel position groups and the spatial frequencydistribution of the print image is higher than any of the correlationcoefficients between each of the spatial frequency distributions of dotpatterns formed on print pixels belonging to each of any of the otherplurality of the pixel position groups that form print images bymutually combining a common print area and the spatial frequencydistribution of the print image, or the halftone process is constitutedso that, at least for the tone level with relatively low dot density,the RMS granularity of the dot pattern formed on the print pixelsbelonging to each of the plurality of pixel position groups is lowerthan the RMS granularity of the dot pattern formed on the print pixelsbelonging to each of any of the other of the plurality of pixel positiongroups that form print images by mutually combining a common print area.

In this way, with this invention, it is acceptable as long as theoptimality for the plurality of pixel position groups to be evaluated iscompensated.

This invention further provides a printing apparatus with the followingaspects. Specifically, a printing apparatus that prints images byforming dots both during forward scan and backward scan of the dotforming head, comprising: a dither matrix for which a threshold value isset for each pixel, dot formation presence or absence decision meansthat receives image data representing the tone value of each pixelconstituting an image and decides the presence or absence of dotformation for each pixel according to the tone value of each of thepixels and to the threshold value set for the pixel positioncorresponding to the dither matrix, and dot forming means that formsdots based on the results of deciding the dot formation presence orabsence, and the dither matrix is a matrix having either blue noisecharacteristics or green noise characteristics for both threshold valuedistribution set for the first pixel position group used for decidingthe presence or absence of dot formation for the pixels for which dotsare formed with either the forward scan or the backward scan of the dotforming head, and the threshold value distribution set for the secondpixel position group excluding the first pixel position group from thedither matrix.

Also, the printing method of this invention corresponding to theprinting apparatus noted above is a printing method that prints imagesby forming dots both during forward scan and backward scan of the dotforming head, comprising: a first step that stores the dither matrix forwhich the threshold values are set for each pixel, a second step thatreceives image data representing the tone value of each pixelconstituting an image and decides the presence or absence of dotformation for each pixel according to the tone value of each of thepixels and to the threshold value set for the pixel positioncorresponding to the dither matrix, and a third step that forms dotsbased on the results of deciding the dot formation presence or absence,and the dither matrix stored at the first step is a matrix having eitherblue noise characteristics or green noise characteristics for boththreshold value distribution set for the first pixel position group usedfor deciding the presence or absence of dot formation for the pixels forwhich dots are formed with either the forward scan or the backward scanof the dot forming head, and the threshold value distribution set forthe second pixel position group excluding the first pixel position groupfrom the dither matrix.

With the printing apparatus and printing method according to theinvention of this application, the presence or absence of dot formationfor each pixel is decided while referring to a dither matrix like thefollowing. Specifically, each pixel position of the dither matrix can beclassified as either a first pixel position or a second pixel position,and the matrix is such that the distribution of the threshold values setfor the first pixel position and the distribution of the thresholdvalues set for the second pixel position all have either blue noisecharacteristics or green noise characteristics. Here, the first pixelposition and second pixel position means pixel positions having arelationship such that when forming dots while moving the dot forminghead back and fort, with one of the pixel positions, dots are formedwith either the forward scan or the backward scan, and with the otherpixel position, dots are formed with the other. Note that just becauseit is said that each pixel position on the dither matrix can beclassified as a first pixel or a second pixel position, doesn'tnecessarily mean that the direction for forming the dot of each pixelposition is fixed to the forward scan or the backward scan.

Also, the distribution of the threshold values having blue noisecharacteristics means the following kind of distribution. Specifically,when dots are generated using a dither matrix having that kind ofthreshold value distribution, dots are generated irregularly, and thespatial frequency component of the set threshold value means thedistribution of a threshold value such as one having the biggestcomponent in the high frequency range with one cycle as two pixels orless. Also, distribution of threshold values having green noisecharacteristics are distributions like the following. Specifically, whendots are generated using a dither matrix having that kind of thresholdvalue distribution, dots are generated irregularly, and the spatialfrequency component of the set threshold value means the distribution ofa threshold value such as one having the biggest component in the mediumfrequency range with one cycle as from two pixels to ten or more pixels.

The detailed principle is described in detail later, but the degradationof image quality that occurs when the dot formation position isdisplaced between forward scan and backward scan when doingbidirectional printing can be greatly suppressed by suitably dispersingdots for both images made only with dots formed during the forward scanand images made only by dots formed during the backward scan. As is wellknown, using a dither matrix having blue noise characteristics or greennoise characteristics, it is possible to suitably disperse dots if thepresence or absence of dot formation is decided for each pixel.Therefore, if a dither matrix such as one having respectively blue noisecharacteristics or green noise characteristics is used for thedistribution of threshold values set for the first pixel positions anddistribution of threshold values set for the second pixel positions, itis possible to suitably disperse dots for both images made only withdots formed during the forward scan and images made only by dots formedduring the backward scan, and thus, it is possible to suppress to aminimum the degradation of image quality when there is displacement ofthe dot formation positions.

Also, with this kind of printing apparatus, it is also possible todecide the presence or absence of dot formation for each pixel whilereferencing the following kind of dither matrix. Specifically, it isalso possible to reference a dither matrix such as one for which, whenthe pixel positions of the matrix are classified into rasters that arepixel positions aligned in the direction in which the dot formation headmoves back and forth, only one or the other of the first pixel positionor the second pixel position is contained within one of the rasters.

By working in this way, even if there is dot formation positionmisalignment between the dot formation head forward scan and backwardscan, within the same raster, dots are formed only of one or the otherof the forward scan or backward scan, and the distance between dots doesnot come too close or too far, so it is possible to suppress degradationof the image quality.

Also, with this kind of dither matrix, it is also possible to alignrasters containing only first pixel positions and rasters containingonly second pixel positions alternately in a direction intersecting withthe raster.

By working in this way, with dots formed during forward scan and dotsformed during backward scan, even if the dot formation positions aredisplaced, the dots are displaced in one direction over consecutiverasters, and it is possible to avoid this from being visible anddegrading the image quality.

Also, the printing apparatus described above forms dots based on thepresence or absence of dot formation decided for each pixel, and whenthe presence or absence of dot formation for each pixel is decided, ifthe focus is on deciding this by referencing a dither matrix havingspecified characteristics, the invention of this application can also beunderstood as the following kind of image processing device and imageprocessing method. Specifically, the image processing device of theinvention of this application is an image processing device thatgenerates control data used for a printing apparatus that prints imagesby forming dots both during forward scan and backward scan of the dotforming head to control the dot formation, comprising: a dither matrixfor which threshold values are set for each pixel, dot formationpresence or absence decision means that receives image data representingthe tone value of each pixel constituting an image and decides thepresence or absence of dot formation for each pixel according to thetone value of each of the pixels and to the threshold value set for thepixel position corresponding to the dither matrix, and control dataoutput means that outputs the results of deciding the dot formationpresence or absence as the control data, and the dither matrix is amatrix having either blue noise characteristics or green noisecharacteristics for both threshold value distribution set for the firstpixel position group used for deciding the presence or absence of dotformation for the pixels for which dots are formed with either theforward scan or the backward scan of the dot forming head, and thethreshold value distribution set for the second pixel position groupexcluding the first pixel position group from the dither matrix.

The method of generating this kind of dither matrix is a method ofgenerating a dither matrix for printing that forms a print image bymutually combining in a common print area the print pixels belonging toeach of the plurality of pixel position groups for which physicaldifferences are assumed during dot formation, comprising: setting of theevaluation function that sets the evaluation function for calculatingthe evaluation value of the dither matrix, preparing that prepares adither matrix as the initial state for storing in each element aplurality of threshold values for deciding the presence or absence ofdot formation for each pixel according to the input tone value, decidingof the storage elements that, while replacing part of the plurality ofthreshold values stored in each element with threshold values stored inother elements, decides the elements in which each threshold value isstored, and outputting of a dither matrix for which the storage elementis decided for all of the plurality of threshold values, the deciding ofthe storage element including: mutual replacing of part of the pluralityof threshold values, calculating of the evaluation value of the dithermatrix for which the threshold value was replaced using the evaluationfunction, and deciding of the storage element for the plurality ofthreshold values according to conformity to a specified criterion of theevaluation value, and the evaluation function is constituted based onthe characteristics of the dot pattern formed on the print pixelsbelonging to each of the plurality of pixel position groups at least forthe tone level with relatively low dot density.

For the dither matrix generating method noted above, it is preferablethat the evaluation function be set to be the RMS granularity of the dotpattern formed on the print pixels belonging to each of the plurality ofpixel position groups for at least the part of the gradations for whichthe dot density is relatively low.

The RMS granularity is an objective measure representing variation inthe dot denseness, and it is capable of doing simple calculation simplywith a smoothing process using a smoothing filter set according to theresolution and with calculation of the standard deviation of the dotformation density, so it is very suitable for optimization processingwhich involves many calculation repetitions. In addition, use of RMSgranularity is because it has the advantage of it being possible to doflexible processing considering human visual sense and visualenvironment according to the design of the smoothing filter incomparison to a fixed process using the human visual sensecharacteristic VTF.

This kind of dither matrix generating method is a method of generating adither matrix for printing that forms a print image by mutuallycombining in a common print area the print pixels belonging to each ofthe plurality of pixel position groups for which physical differencesare assumed during dot formation, comprising: setting of the evaluationfunction that sets the evaluation function for calculating theevaluation value of the dither matrix, preparing that prepares a dithermatrix as the initial state for storing in each element a plurality ofthreshold values for deciding the presence or absence of dot formationfor each pixel according to the input tone value, deciding of thestorage elements that, while replacing part of the plurality ofthreshold values stored in each element with threshold values stored inother elements, decides the elements in which each threshold value isstored, and outputting of a dither matrix for which the storage elementis decided for all of the plurality of threshold values, the deciding ofthe storage element including: mutual replacing of part of the pluralityof threshold values, calculating of the overall evaluation value that isthe evaluation value of the dither matrix for which the threshold valuewas replaced using the evaluation function, and deciding of the storageelement for the plurality of threshold values according to conformity toa specified criterion of the overall evaluation value and each of thegroup evaluation values.

Also, the image processing method of this invention corresponding to theimage processing device noted above is an image processing method thatgenerates control data used for a printing apparatus that prints imagesby forming dots both during forward scan and backward scan of the dotforming head to control the dot formation, comprising: step (A) thatstores the dither matrix for which threshold values are set for eachpixel, step (B) that receives image data representing the tone value ofeach pixel constituting an image and decides the presence or absence ofdot formation for each pixel according to the tone value of each of thepixels and to the threshold value set for the pixel positioncorresponding to the dither matrix, and step (C) that outputs theresults of deciding the dot formation presence or absence as the controldata, and the dither matrix stored at the step (A) is a matrix havingeither blue noise characteristics or green noise characteristics forboth threshold value distribution set for the first pixel position groupused for deciding the presence or absence of dot formation for thepixels for which dots are formed with either the forward scan or thebackward scan of the dot forming head, and the threshold valuedistribution set for the second pixel position group excluding the firstpixel position group from the dither matrix.

For this image processing device and image processing method as well,the same as with the previously describe printing apparatus and printingmethod, when deciding the presence or absence of dot formation for eachpixel, the following kind of dither matrix is referenced. Specifically,referenced is a matrix such as one for which each of the pixel positionsof the dither matrix can be classified as either a first pixel positionor a second pixel position, and both the distribution of the thresholdvalues set for the first pixel position and the distribution of thethreshold values set for the second pixel position have either bluenoise characteristics or green noise characteristics. When an image isprinted using control data generated referencing this kind of dithermatrix, even if the dot formation positions are displaced between thedot formation head forward scan and backward scan, it is possible tosuppress to a minimum the degradation of the image quality due to that,and to print a high image quality image.

Furthermore, the invention of this application can also be realizedusing a computer by reading into a computer a program for realizing theprinting method or the image processing method described above.Therefore, this invention also includes aspects as the following kind ofprogram or as a recording medium on which is recorded the program.Specifically, the program of the invention of this applicationcorresponding to the printing method described above is a program thatrealizes using a computer a method that prints images by forming dotsboth during forward scan and backward scan of the dot forming head,realizing using the computer: a first function that stores the dithermatrix for which threshold values are set for each pixel, a secondfunction that receives image data representing the tone value of eachpixel constituting an image and decides the presence or absence of dotformation for each pixel according to the tone value of each of thepixels and to the threshold value set for the pixel positioncorresponding to the dither matrix, and a third function that forms dotsbased on the results of deciding the dot formation presence or absence,and the dither matrix stored by the first function is a matrix havingeither blue noise characteristics or green noise characteristics forboth threshold value distribution set for the first pixel position groupused for deciding the presence or absence of dot formation for thepixels for which dots are formed with either the forward scan or thebackward scan of the dot forming head, and the threshold valuedistribution set for the second pixel position group excluding the firstpixel position group from the dither matrix.

Also, the recording medium of the invention of this applicationcorresponding to the program noted above is a recording medium on whichis recorded a computer readable program that prints images by formingdots both during forward scan and backward scan of the dot formationhead, recording functions realized using the computer of: a firstfunction that stores the dither matrix for which threshold values areset for each pixel, a second function that receives image datarepresenting the tone value of each pixel constituting an image anddecides the presence or absence of dot formation for each pixelaccording to the tone value of each of the pixels and to the thresholdvalue set for the pixel position corresponding to the dither matrix, anda third function that forms dots based on the results of deciding thedot formation presence or absence, and the dither matrix stored by thefirst function is a matrix having either blue noise characteristics orgreen noise characteristics for both threshold value distribution setfor the first pixel position group used for deciding the presence orabsence of dot formation for the pixels for which dots are formed witheither the forward scan or the backward scan of the dot forming head,and the threshold value distribution set for the second pixel positiongroup excluding the first pixel position group from the dither matrix.

Also, the program of the invention of this application corresponding tothe image processing method described above is a program that realizesusing a computer a method that generates control data used for aprinting apparatus that prints images by forming dots both duringforward scan and backward scan of the dot forming head to control thedot formation, realizing using the computer: function (A) that storesthe dither matrix for which threshold values are set for each pixel,function (B) that receives image data representing the tone value ofeach pixel constituting an image and decides the presence or absence ofdot formation for each pixel according to the tone value of each of thepixels and to the threshold value set for the pixel positioncorresponding to the dither matrix, and function (C) that outputs theresults of deciding the dot formation presence or absence as the controldata, and the dither matrix stored by the function (A) is a matrixhaving either blue noise characteristics or green noise characteristicsfor both threshold value distribution set for the first pixel positiongroup used for deciding the presence or absence of dot formation for thepixels for which dots are formed with either the forward scan or thebackward scan of the dot forming head, and the threshold valuedistribution set for the second pixel position group excluding the firstpixel position group from the dither matrix.

Also, the recording medium of the invention of this applicationcorresponding to the program noted above is a recording medium on whichis recorded to be readable on the computer a program that generatescontrol data used for a printing apparatus that prints images by formingdots both during forward scan and backward scan of the dot forming headto control the dot formation, recording a program realized using thecomputer of: function (A) that stores the dither matrix for whichthreshold values are set for each pixel, function (B) that receivesimage data representing the tone value of each pixel constituting animage and decides the presence or absence of dot formation for eachpixel according to the tone value of each of the pixels and to thethreshold value set for the pixel position corresponding to the dithermatrix, and function (C) that outputs the results of deciding the dotformation presence or absence as the control data, and the dither matrixstored by the function (A) is a matrix having either blue noisecharacteristics or green noise characteristics for both threshold valuedistribution set for the first pixel position group used for decidingthe presence or absence of dot formation for the pixels for which dotsare formed with either the forward scan or the backward scan of the dotforming head, and the threshold value distribution set for the secondpixel position group excluding the first pixel position group from thedither matrix.

If this kind of program or program recorded on a recording medium isread into a computer and the various functions described above arerealized using the computer, even when the dot formation positions aredisplaced between the dot formation head forward scan and backward scan,it is possible to suppress to a minimum the effect due to this. Becauseof this, it is possible to rapidly print high image quality images andalso possible to simplify the mechanism and control for adjusting thedot formation position with the forward scan and backward scan.

Finally, the parent of the present application claims priority based onJapanese Patent Application No. 2005-032771 filed on Feb. 9, 2005,Japanese Patent Application No. 2005-171290 filed on Jun. 10, 2005, andJapanese Patent Application No. 2005-210792 filed on Jul. 21, 2005, thedisclosures of which are herein incorporated by reference in theirentirety for all purposes.

1. A printing apparatus that performs printing on a print medium, comprising: a dot data generator that performs a halftone process on image data representing a tone value of each of pixels constituting an original image to determine a status of dot formation on each of print pixels of a print image to be formed on the print medium, for generating dot data representing the determined status of dot formation, and a print image generator that forms a dot on each of the print pixels for generating a print image according to the dot data, wherein the print image is formed by mutually combining print pixels belonging to each of a plurality of pixel position groups for which a physical difference is assumed at a formation of dots by the print image generator, in a common print area, and the halftone process is configured to generate the dot data, so that a pattern of dots belonging to each of the plurality of pixel position groups has either blue noise characteristics or green noise characteristics.
 2. The printing apparatus according to claim 1, wherein the physical difference includes a displacement of timing of dot formation for each of the plurality of pixel position groups.
 3. The printing apparatus according to claim 2, wherein the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 4. The printing apparatus according to claim 1, wherein the physical difference includes a relative shift of dot position for each of the plurality of pixel position groups.
 5. The printing apparatus according to claim 4, wherein the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 6. The printing apparatus according to claim 1, wherein the halftone process is further configured to provide each dot pattern with specified characteristics, each dot pattern including dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups.
 7. The printing apparatus according to claim 4, wherein the halftone process is further configured to provide a hypothetical print image on an assumption of none of the relative shift of dot position with specified characteristics.
 8. The printing apparatus according to claim 4, wherein the halftone process is further configured to provide a plurality of hypothetical print images with specified characteristics, the plurality of hypothetical print images including a first hypothetical print image on an assumption of the relative shift of dot position and a second hypothetical print image on an assumption of none of the relative shift of dot position.
 9. The printing apparatus according to claim 1, wherein the print image generator has a printing head and generates a print image by forming dots on each of the print pixels during forward scan and backward scan of the printing head, while performing a main scan of the printing head, and the plurality of pixel position groups includes a first pixel position group for which dots are formed during the forward scan of the printing head and a second pixel position group for which dots are formed during the backward scan of the printing head.
 10. The printing apparatus according to claim 1, wherein the print image generator has a printing head and generates a print image by forming dots on each of the print pixels while repeating a pass of the printing head N times according to the dot data, wherein N is an integer of 2 or more and is determined by a nozzle pitch, and wherein each of the plurality of pixel position groups is formed by two passes.
 11. The printing apparatus according to claim 1, wherein the print image generator has a plurality of printing heads and generates a print image by forming dots on each of the print pixels according to the dot data, while performing a main scan of the plurality of printing heads, and the plurality of pixel position groups includes a plurality of pixel position groups for which each of the plurality of printing heads is in charge of the dot formation of each of the plurality of pixel position groups.
 12. The printing apparatus according to claim 1, wherein the print image generator has a plurality of printing heads and generates a print image by forming dots on each of the print pixels according to the dot data, while performing a sub-scan of the print medium, and the plurality of pixel position groups includes a plurality of pixel position groups for which each of the plurality of printing heads is in charge of the dot formation of each of the plurality of pixel position groups.
 13. The printing apparatus according to claim 9, wherein the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient.
 14. The printing apparatus according to claim 9, wherein the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 15. The printing apparatus according to claim 9, wherein the halftone process is configured such that any first correlation coefficients are higher than any of second correlation coefficients at least for tone levels with relatively low dot density, wherein the first correlation coefficients are coefficients between each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distribution of the print image, and the second correlation coefficients are correlation coefficients between each of spatial frequency distributions of dot patterns formed on print pixels belonging to each of any of other of the plurality of the pixel position groups that form print images by mutually combining in a common print area and the spatial frequency distribution of the print image.
 16. The printing apparatus according to claim 9, wherein the halftone process is configured such that a RMS granularity of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups is lower than the RMS granularity of dot patterns formed on the print pixels belonging to each of any of other of the plurality of pixel position groups that form print images by mutually combining in a common print area, at least for tone levels with relatively low dot density.
 17. A printing method of printing on a print medium, comprising: performing a halftone process on image data representing a tone value of each of pixels constituting an original image to determine a status of dot formation on each of print pixels of a print image to be formed on the print medium, for generating dot data representing the determined status of dot formation, and forming a dot on each of the print pixels for generating a print image according to the dot data, wherein the print image is formed by mutually combining a plurality of pixel position groups for which a physical difference is assumed at a formation of dots by the print image generator, in a common print area, and the halftone process is configured to generate the dot data, so that a pattern of dots belonging to each of the plurality of pixel position groups has either blue noise characteristics or green noise characteristics.
 18. The printing method according to claim 17, wherein the physical difference includes a displacement of timing of dot formation for each of the plurality of pixel position groups, and the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 19. The printing method according to claim 17, wherein the physical difference includes a relative shift of dot position for each of the plurality of pixel position groups, and the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 20. A computer program product for causing a computer to generate print data to be supplied to a print image generator for generating a print image by forming dots on a print medium, the computer program product comprising: a non-transitory computer readable medium; and a computer program stored on the non-transitory computer readable medium, the computer program comprising a program for causing the computer to perform a halftone process on image data representing a tone value of each of pixels constituting an original image to determine a status of dot formation on each of print pixels of a print image to be formed on the print medium, for generating dot data representing the determined status of dot formation, wherein the print image is formed by mutually combining print pixels belonging to each of a plurality of pixel position groups for which a physical difference is assumed at a formation of dots by the print image generator, in a common print area, and the halftone process is configured to generate the dot data, so that a pattern of dots belonging to each of the plurality of pixel position groups has either blue noise characteristics or green noise characteristics.
 21. The computer program product according to claim 20, wherein the physical difference includes a displacement of timing of dot formation for each of the plurality of pixel position groups, and the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 22. The computer program product according to claim 20, wherein the physical difference includes a relative shift of dot position for each of the plurality of pixel position groups, and the halftone process is configured such that each of spatial frequency distributions of dot patterns formed on the print pixels belonging to each of the plurality of pixel position groups and spatial frequency distributions of the print image have a mutually positive correlation coefficient of at least 0.7.
 23. A printing apparatus that performs printing on a print medium, comprising: a dot data generator that performs a halftone process on image data representing a tone value of each of pixels constituting an original image to determine a status of dot formation on each of print pixels of a print image to be formed on the print medium, for generating dot data representing the determined status of dot formation, and a print image generator equipped with a printing head configured to generate a print image by forming dots on each of the print pixels during forward scan and backward scan of the printing head, while performing a main scan of the printing head, wherein the print image is formed by mutually combining print pixels belonging to each of a plurality of pixel position groups including a first pixel position group for which dots are formed during the forward scan of the printing head and a second pixel position group for which dots are formed during the backward scan of the printing head, the halftone process is configured to generate the dot data, so that a pattern of dots belonging to each of the plurality of pixel position groups has either blue noise characteristics or green noise characteristics, and the halftone process is configured such that any of mutual correlation coefficients between a plurality of spatial frequency distributions consisting of spatial frequency distributions of dot patterns formed on the print pixels belonging to the first pixel position group, spatial frequency distributions of dot patterns formed on the print pixels belonging to the second pixel position group, and spatial frequency distributions of the print image is at least 0.7, at least for the tone level with relatively low dot density.
 24. A printing method of printing on a print medium, comprising: performing a halftone process on image data representing a tone value of each of pixels constituting an original image to determine a status of dot formation on each of print pixels of a print image to be formed on the print medium, for generating dot data representing the determined status of dot formation, and forming dots on each of the print pixels during forward scan and backward scan of a printing head, while performing a main scan of the printing head, for generating a print image according to the dot data, wherein the print image is formed by mutually combining print pixels belonging to each of a plurality of pixel position groups including a first pixel position group for which dots are formed during the forward scan of the printing head and a second pixel position group for which dots are formed during the backward scan of the printing head, the halftone process is configured to generate the dot data, so that a pattern of dots belonging to each of the plurality of pixel position groups has either blue noise characteristics or green noise characteristics, and the halftone process is configured such that any of mutual correlation coefficients between a plurality of spatial frequency distributions consisting of spatial frequency distributions of dot patterns formed on the print pixels belonging to the first pixel position group, spatial frequency distributions of dot patterns formed on the print pixels belonging to the second pixel position group, and spatial frequency distributions of the print image is at least 0.7, at least for the tone level with relatively low dot density. 